Process for preparing monohalogenated cyclobutarenes

ABSTRACT

Monohalogenated cyclobutarenes are prepared by halogenating a cyclobutarene in the presence of an organic complexing agent, an acid scavenger, or water. Faster reaction rates highly selective to monohalogenated cyclobutarenes are obtained without conventional heavy metal or halogen catalysts.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of Application Ser. No. 064,714, filedJune 22, 1987.

BACKGROUND OF THE INVENTION

This invention relates to a process for preparing halogenated organiccompounds. More specifically, it relates to a process for preparingmonohalogenated cyclobutarenes.

Monohalogenated cyclobutarenes are intermediates for the preparation ofhigh performance monomeric and polymeric compositions for theelectronics and aerospace industries. U.S. Pat. No. 4,540,763 disclosesthat monobrominated cyclobutarenes can be processed to preparepoly(cyclobutarene) polymeric compositions. These compositions possessthermal stability at high temperatures, as well as chemical resistanceand low sensibility to water.

Processes for preparing monohalogenated cyclobutarenes are difficultbecause multiple halogenation reactions occur and the strainedcyclobutane ring of the cyclobutarene is easily susceptible toring-opening side reactions (see J. B. F. Lloyd et al., Tetrahedron, 20,pp. 2185-94 (1964)). U.S. Pat. No. 4,540,763 discloses a process forpreparing monobrominated cyclobutarenes which involves diluting acyolobutarene in acetic acid and then contacting the solution withpyridinium perbromide hydrobromide in the presence of a mercuric acetatecatalyst. The reaction occurs over a four day period and usesapproximately 300 percent excess brominating agent. J. B. F. Lloyd etal., Tetrahedron, 21, pp. 245-54, (1965), disclose a process forpreparing monobrominated benzocyclobutene which involves dilutingbenzocyclobutene in a 95 percent aqueous solution of acetic acid andthen contacting the solution with molecular bromine in the presence ofan iodine catalyst. The yield of monobrominated benzocyclobutene is 78percent after 48 hours. Unfortunately, both of these processes requirelarge quantities of brominating agent to complete a very slowbromination reaction. Also, both processes require either a heavy metalcatalyst or a halogen catalyst. The residual catalyst that inevitablyfinds its way into the final product is detrimental for electronics andaerospace industry applications. Furthermore, these catalysts createenvironmental problems related to their disposal.

Therefore, it would be desirable to have a process for preparingmonohalogenated cyclobutarenes that does not require a halogen catalystor a heavy metal catalyst. It would also be desirable to have a processproviding a faster halogenation reaction highly selective tomonohalogenated cyclobutarenes without requiring excessive quantities ofhalogenating agent.

SUMMARY OF THE INVENTION

This invention is a method of preparing monohalogenated cyclobutarenesconsisting essentially of halogenating a cyclobutarene in the presenceof an organic complexing agent, an acid scavenger, or water.Surprisingly, reaction rates faster than the rates disclosed in theprior art are achieved by the method of this invention without requiringa catalyst. In addition, the reaction is highly selective tomonohalogenated cyclobutarenes and neither requires excessive quantitiesof halogenating agent nor creates an environmental problem related tothe disposal of the catalysts.

The monohalogenated cyclobutarenes of this invention are useful asintermediates for the preparation of high performance monomeric andpolymeric compositions for the electronics industry.

DETAILED DESCRIPTION OF THE INVENTION

As the term is used herein, "cyclobutarene" refers to a compoundcontaining at least one aromatic ring to which is fused one or morecyclobutane rings or one or more substituted cyclobutane rings. Anaromatic ring contains (4N + 2)π electrons as described in Morrison andBoyd, Organic Chemistry, 3rd Edition, (1973). Suitable compoundscontaining at least one aromatic ring include benzene, naphthalene,biphenyl, binaphthyl, phenanthrene, anthracene, and diphenylbenzene. Thearomatic ring of the cyclobutarene can be substituted with groups stableto the bromination reaction, including but not limited to groups such asmethyl, methoxy, and acetate. Heterocyclic compounds such as pyridineand picoline are also included. Preferred compounds are benzene,naphthalene, and biphenyl. The most preferred compound containing atleast one aromatic ring is benzene. Therefore, the most preferredcyclobutarene is benzocyclobutene.

As disclosed in U.S. Pat. No. 4,570,011, cyclobutarenes useful in thisinvention can be prepared by dissolving an ortho alkyl halomethylaromatic hydrocarbon, such as ortho methylchloromethylbenzene, in aninert solvent, and then pyrolyzing the solution under suitable reactionconditions.

"Halogenating" refers to the introduction of a halogen into an organiccompound by treating the compound with a halogenating agent. Suitablehalogenating agents useful in this invention are those compounds whichare capable of reacting with the aromatic ring of the cyclobutarene tobreak the carbon-hydrogen bond and to form a carbon-halogen bond underthe reaction conditions Halogenating agents useful for halogenatingcyclobutarenes are disclosed in H. P. Braendlin et al., Friedel-Craftsand Related Reactions, Vol. III, Chapter 46, pp. 1517-1593, John Wiley &Sons, New York (1964): Wagner et al., Synthetic Organic Chemistry, pp.98-147, John Wiley & Sons, New York (1965); and March, Advanced OrganicChemistry, 3rd ed., pp. 476-479, John Wiley & Sons, New York (1985).Each of these references is incorporated by reference herein. Preferredhalogenating agents are brominating and chlorinating agents. The mostpreferred halogenating agent is a brominating agent.

The brominating agents that can be employed in this invention caninclude molecular bromine, bromine chloride, pyridinium perbromidehydrobromide, dioxane dibromide, and N-bromosuccinimide. Preferredbrominating agents include molecular bromine and bromine chloride. Themost preferred brominating agent is molecular bromine.

Preferred chlorinating agents include molecular chlorine,N-chlorosuccinimide, and t-butyl hypochlorite. Preferred iodinatingagents include molecular iodine and iodine monochloride.

The monohalogenated cyclobutarenes useful in this invention are preparedby halogenating a cyclobutarene. The term "monohalogenated" refers tothe replacement of one hydrogen atom on the aromatic ring with onehalogen atom. The products produced from the halogenation of thecyolobutarene include not only the monohalogenated cyclobutarenes butalso small quantities of hydrogen halide, unreacted halogenating agentand undesirable side reaction products. The hydrogen halide can eitherdissolve in the reaction mixture or evolve from the reaction mixture asa gas.

The organic complexing agents that improve the selectivity of thereaction to monohalogenated cyclobutarenes are organic compounds thatwill donate electrons to form donor-acceptor adducts with the unreactedhalogenating agent and the hydrogen halide produced during the reaction.The adduct formed reduces the reactivity of the halogenating agent andhydrogen halide with the cyclobutane ring of the cyclobutarene andtherefore reduces formation of undesirable side products A. J. Downs etal., Comprehensive Inorganic Chemistry, Chapter 26, pp. 1196-1197 andpp. 1201-1209, New York, N.Y., (1973), discuss the crystalline structureof halogen adducts based on X-ray diffraction studies. They describeorganic compounds which form halogen adducts and the factors influencingtheir stability. They also describe the relative capacities of organiccompounds to donate electrons. Preferably, the organic complexing agenthas an electron donor capacity equal to or slightly greater than theelectron donor capacity of the cyclobutarene.

Suitable organic complexing agents include aliphatic alcohols and diolshaving less than 10 carbon atoms, such as methanol, isobutyl alcohol,and ethylene glycol: aliphatic polymeric diols having an averagemolecular weight ranging from about 100 to about 15,000, such as thecommercial grades of polyethylene glycol and polypropylene glycol:saturated aliphatic ethers having less than 10 carbon atoms, such asethylene glycol ethyl ether and tripropylene glycol methyl ether;saturated cyclic ethers such as dioxane and 12-crown-4 ether: saturatedaliphatic carboxylic acids and their anhydrides having less than 10carbon atoms, such as acetic acid and acetic anhydride; other complexingagents such as dimethyl formamide and dimethyl sulfoxide: and mixturesof these organic complexing agents. Preferred organic complexing agentsare methanol and ethylene glycol ethyl ether. The most preferred organiccomplexing agent is methanol.

Other organic complexing agents that improve the selectivity of thereaction to monohalogenated cyclobutarenes include saturated quaternaryammonium salts, such as tetraalkylammonium salts and trialkylaminesalts. Although these compounds do not donate electrons to formdonor-acceptor adducts, their effectiveness as complexing agents hasbeen demonstrated.

The Dictionary of Scientific and Technical Terms, McGraw-Hill, SecondEdition (1978) defines a scavenger as "a substance added to a mixture orother system to remove or inactivate impurities". Acid scavengers usefulin this invention remove or inactivate hydrogen halide produced duringthe halogenation by reacting with the hydrogen halide to form a sideproduct. The scavenger does not react with the cyclobutarene.Preferably, the scavenger reacts readily with hydrogen halide but doesnot react readily with the halogenating agent to prevent thehalogenation of the cyolobutarene. The acid scavenger can be organic orinorganic.

Suitable organic acid scavengers include epoxides having less than 10carbon atoms, such as ethylene oxide, propylene oxide, epichlorohydrin,and epibromohydrin: aliphatic tertiary alcohols having less than 10carbon atoms, such as tertiary butyl alcohol: aliphatic primary,secondary and tertiary amines, such as ethylamine, diethylamine, andtriethylamine; heterocyclic compounds such as pyridine and picoline, andtriarylphosphines such as triphenylphosphine. The preferred scavengersare the epoxides having less than 10 carbon atoms and the tertiaryamines. The most preferred epoxide is epichlorohydrin and the mostpreferred tertiary amine is triethylamine.

Suitable inorganic acid scavengers include alkali metal and alkali earthmetal salts of alcohols and carboxylic acids, such as sodium methylate,sodium ethylate, and sodium acetate: alkali metal and alkali earth metalbases, such as sodium hydroxide and calcium hydroxide; and carbonatesand bicarbonates of alkali metal and alkali earth metals, such as sodiumbicarbonate and potassium carbonate.

When the cyolobutarene is halogenated in the presence of water, thewater acts in a manner similar to that of the organic complexing agentby forming donor-acceptor adducts with the unreacted halogenating agentand the hydrogen halide.

In a preferred embodiment of this invention, the solubility of hydrogenhalide produced during halogenation in the reaction mixture is reduced.The reduced solubility will increase the quantity of hydrogen halidethat will evolve from the reaction mixture as a gas. Since more hydrogenhalide will evolve from the reaction mixture as a gas, there will beless hydrogen halide in the reaction mixture that can react with thecyclobutane ring of the cyclobutarene to produce undesirable sideproducts. Therefore, an increased selectivity of monohalogenatedcyclobutarene will result.

One method of reducing the solubility of hydrogen halide in the reactionmixture is to dilute the cyclobutarene in an appropriate nonreactingdiluent before halogenation. Appropriate diluents are those in which thesolubility of hydrogen halide is low. Ahmed et al., Journal of AppliedChemistry, 20., pp. 109-116, (April 1970), disclose the solubilities ofhydrogen halides in various diluents. Suitable diluents that can beemployed in this invention include methylene chloride, chloroform,carbon tetrachloride, ethylene dichloride, bromochloromethane, andhexane. The preferred diluents are methylene chloride, chloroform, andbromochloromethane. The most preferred diluent is methylene chloride.

Certain organic complexing agents can also act as appropriatenonreacting diluents. Examples of such organic complexing agents includeacetic acid, methanol, and water.

The mole ratio of the cyclobutarene to the complexing agent or wateremployed in the practice of this invention can range from about 0.001:1to about 100:1. A more preferable range is from about 0.005:1 to about70:1. The most preferable range is from about 0.05:1 to about 6.0:1. Themole ratio of the cyclobutarene to the scavenger employed in thepractice of this invention can range from about 0.1:1 to about 100:1. Amore preferable range is from about 0.3:1 to about 20:1. The mostpreferable range is from about 0.5:1 to about 2.0:1.

If a diluent is employed to dilute the cyclobutarene beforehalogenation, the weight ratio of the diluent to the cyclobutarene canrange from about 0 1:1 to about 100:1. A more preferable range is fromabout 0.5:1 to about 20:1. The mole ratio of the halogenating agent tothe cyclobutarene can range from about 0.1:1 to about 2.0:1. A morepreferable range is from about 0.90:1 to about 1.10:1.

The operating temperature and pressure of the reaction system arelimited solely by practical considerations. The temperature can rangefrom the freezing point to the boiling point of the reaction mixture.Preferably, the operating temperature ranges from about 25° C. to about60° C. Although the halogenation reaction will proceed at both high andlow operating pressures, it is preferable to run as close to atmosphericpressure as possible because higher pressures will increase thesolubility of the hydrogen halide in the reaction system and thereforegenerate more side reactions. Also, high operating pressures necessitatethe use of more expensive pressure rated equipment.

In a preferred embodiment of this invention, the halogenating agent isadded continuously or periodically to the reaction mixture to controlthe evolution of gaseous hydrogen halide. By controlling the evolutionof the gaseous hydrogen halide, the operating pressure of the system canbe maintained as close to atmospheric pressure as possible.

The halogenation reaction proceeds almost instantaneously when thehalogenating agent contacts the cyclobutarene. In most instances, therequired reaction time depends on the rate of addition of thehalogenating agent to the reaction system. The rate of addition of thehalogenating agent depends on the ability of the system to remove thegaseous hydrogen halide and the design pressure of the reactor.

The selectivity of the reaction to monohalogenated cyclobutarenesdecreases with conversion because the monohalogenated cyclobutarenesprepared from the halogenation can react further with the reactionmixture to form undesirable side products. Advantageously, themonohalogenated cyclobutarenes are separated quickly from the reactionmixture. In preferred embodiments of this invention, the selectivitywill range from about 75 mole percent to about 95 mole percent.Selectivity is defined as the mole percentage of the reactedcyclobutarene that forms monohalogenated cyclobutarenes.

After the halogenation reaction, the monohalogenated cyclobutarenes caneasily be separated from the side products produced by the reaction. Onemethod of separation is to fractionally distill all of the impuritiesfrom the reaction system. Another method of separation involves addingan aqueous solution of a reducing agent, such as sodium metabisulfite,to neutralize the residual halogenating agent and to extract thehydrogen halide from the organic phase of the reaction mixture to theaqueous phase. The aqueous phase can then be physically separated fromthe organic phase and then the organic phase can be fractionallydistilled to recover the monohalogenated cyclobutarenes. Preferably, therecovered monohalogenated cyclobutarenes have a purity of at least 97percent by weight.

The recovered monohalogenated cyclobutarenes are useful intermediateswhich can be processed to prepare poly(cyclobutarene) monomeric andpolymeric compositions. U.S. Pat. No. 4,540,763 discloses methods ofpreparing these compositions from monobrominated cyclobutarenes. Thepolymeric compositions have excellent thermal stability at hightemperatures, good chemical resistance to most industrial solvents, anda low sensitivity to water. These properties are highly desirable forapplications in the electronics and aerospace industries. The followingexamples are illustrative and are not intended to limit the scope ofthis invention. All percentages are mole percent unless otherwiseindicated.

EXAMPLES EXAMPLE 1

2005 grams (g) Benzocyclobutene (19.25 moles), 2000 g methylene chloride(23.55 moles) and 200 g methanol (6.24 moles) are charged to a jacketed,8 liter cylindrical 3-neck round bottom reactor equipped with amechanical stirrer, a digital thermocouple, and a reflux condenserconnected to a caustic scrubber. The mixture is heated to 40° C. byrecirculating an aqueous solution of ethylene glycol from a constanttemperature bath through the jacket. 3275 g Bromine (20.49 moles) arefed to the reactor at a constant flow rate of 728 g/hr. During theaddition, the temperature increases to a range between 48° C. and 57.5°C. and reflux is observed. A sample of the reaction mixture is takeneach hour for 4 hours. Another sample is taken after 4 hours and 30minutes when all of the bromine has been fed to the reactor. Theresidual bromine of each sample is neutralized with the requisite amountof an aqueous solution of sodium metabisulfite. Each organic layer isseparated and analyzed using a capillary gas chromatograph to determineits composition. A final sample of the reaction mixture is taken after 5hours and 30 minutes. It is washed with aqueous sodium metabisulfite andthe organic layer is separated and analyzed in a similar manner. Theanalysis of each sample is shown in Table I.

                                      TABLE I                                     __________________________________________________________________________                Mono-        Multi-                                                    Unreacted                                                                            brominated                                                                           2-Bromo-                                                                            brominated                                           Reaction                                                                           Benzo- Benzo- phenethyl                                                                           Benzo- Phenethyl                                     Time cyclobutene                                                                          cyclobutenes                                                                         Bromide                                                                             cyclobutene                                                                          Bromide                                                                             Selectivity                             (Hours)                                                                            (Percent)                                                                            (Percent)                                                                            (Percent)                                                                           (Percent)                                                                            (Percent)                                                                           (Percent)                               __________________________________________________________________________    1.0  84.9   13.7   1.5   0      0     91                                      2.0  58.4   36.9   4.6   0      0     89                                      3.0  38.1   54.7   6.7   0.2    0.2   88                                      4.0  19.2   70.1   8.9   1.1    0.6   87                                      4.5* 10.0   77.0   10.2  1.9    0.9   86                                      5.5  4.4    81.0   10.9  2.7    1.1   85                                      __________________________________________________________________________

Table I indicates that a significantly improved selectivity of thereaction to monobrominated benzocyclobutenes is obtained by the methodof this invention without the use of the catalysts of the prior art.Table I also indicates high selectivities are achieved at much fasterreaction rates than the rates achieved by the prior art.

EXAMPLE 2

100.95 g Benzocyclobutene (0.969 moles), 115.52 g methylene chloride(1.36 moles) and 6.00 g methanol (0.187 moles) are charged to the samereactor as that of Example 1 equipped with a 500 ml dropping funnel. Themixture is heated to 40° C. 163.4 g Bromine (1.022 moles) are addeddropwise to the reaction mixture through the dropping funnel. During theaddition, the temperature increases to 44.2° C. and reflux is observed.After 78 minutes the addition of bromine is completed. After 16 hours,the residual bromine of the reaction mixture is neutralized with 200 mlof an aqueous solution containing 10 g of sodium metabisulfite. Theorganic layer is separated and analyzed using a capillary gaschromatograph. The analysis indicates that the product contains 3.7percent unreacted benzocyclobutene, 81.2 percent monobrominatedbenzocyclobutenes, 6.5 percent 2-bromophenethylbromide, 8.4 percentmultibrominated benzocyclobutenes and less than 0.3 percent phenethylbromide.

EXAMPLE 3

1.6 g Bromine (104 percent theoretical) are added to a solutioncontaining 1 g benzocyclobutene and 0.1 g methanol at room temperature.After 12 hours a sample of the reaction mixture is washed with aqueoussodium metabisulfite. The organic layer is separated and analyzed usinga capillary gas chromatograph. The analysis indicates that the productcontains 24.8 percent benzocyclobutene, 56.5 percent monobrominatedbenzocyclobutenes, 9.1 percent 2-bromophenethyl bromide, 9.3 percentmultibrominated benzocyclobutenes, and 0.3 percent phenethyl bromide.

EXAMPLE 4

In each of a series of runs, 1.6 g bromine are added to a solutioncontaining 4 g methylene chloride, 1 g benzocyclobutene and 0.1 g of oneof several selected complexing agents (or water) at room temperature.After 12 hours a sample of the reaction mixture is washed with aqueoussodium metabisulfite. The organic layer is separated and analyzed usinga capillary gas chromatograph to determine the percent conversion andthe percent selectivity. The conversion and selectivity are compared toa first run in which neither the complexing agent (or water) normethylene chloride are added and a second run in which the complexingagent (or water) is not added. Percent conversion is defined as the molepercentage of benzocyclobutene that reacts. The results are shown inTable II.

                  TABLE II                                                        ______________________________________                                        Complexing             Conversion                                                                              Selectivity                                  Agent(or Water)                                                                          Diluent     (Percent) (Percent)                                    ______________________________________                                        None*      None        92.3      71                                           None*      Methylene   83.1      76                                                      chloride                                                           Methanol   Methylene   96.0      86                                                      chloride                                                           Water      Methylene   90.3      81                                                      chloride                                                           Ethyl Glycol                                                                             Methylene   87.7      87                                           Ethyl Ether                                                                              chloride                                                           Glacial Acetic                                                                           Methylene   94.5      81                                           Acid       chloride                                                           Tetra(n-butyl)                                                                           Methylene   92.8      83                                           Ammonium   Chloride                                                           Hydrogen Sulfate                                                              ______________________________________                                         *Not an embodiment of this invention.                                    

Table II indicates that a high selectivity of the reaction tomonobrominated benzocyclobutenes is obtained by the method of thepresent invention using various complexing agents or water. Theselectivities of the two runs obtained without the complexing agent (orwater) are poor relative to the selectivities obtained according to thepresent invention.

EXAMPLE 5

1.6 g Bromine are added to a solution containing 1 g benzocyclobuteneand 4 g of methanol at room temperature. After 12 hours, a sample of thereaction mixture is washed with aqueous sodium metabisulfite. Theorganic layer is separated and analyzed using a capillary gaschromatograph to determine the percent conversion and the percentselectivity. The experiment is repeated replacing the 4 g of methanolwith 4 g of water. The results are shown in Table III.

                  TABLE III                                                       ______________________________________                                        Complexing             Conversion                                                                              Selectivity                                  Agent (or Water)                                                                         Diluent     (Percent) (Percent)                                    ______________________________________                                        Methanol   None        50.5      85                                           Water      None        92.0      81                                           ______________________________________                                    

Table III indicates that a high selectivity of the reaction tomonobrominated benzocyclobutenes is obtained without the use of adiluent.

EXAMPLE 6

The procedure of Example 4 is followed, except that the methylenechloride diluent is replaced with various diluents listed in Table IVand the complexing agent employed is methanol. The results are shown inTable IV.

                  TABLE IV                                                        ______________________________________                                        Complexing             Conversion                                                                              Selectivity                                  Agent    Diluent       (Percent) (Percent)                                    ______________________________________                                        Methanol 95 percent    73.0      82                                                    Acetic Acid                                                          Methanol Chloroform    88.2      86                                           Methanol Carbon        82.5      80                                                    Tetrachloride                                                        Methanol Ethylene      94.9      81                                                    Dichloride                                                           Methanol Bromochloro-  87.7      84                                                    methane                                                              Methanol Hexane        80.9      81                                           Methanol Water         83.0      77                                           ______________________________________                                    

Table IV indicates that a high selectivity of the reaction tomonobrominated benzocyclobutenes is still obtained using variousdiluents other than methylene chloride.

EXAMPLE 7

The procedure of Example 4 is followed, except that the complexingagents (or water) are replaced with various scavengers listed in TableV. The results are shown in Table V.

                  TABLE V                                                         ______________________________________                                                Molar                                                                         ratio                                                                         of                                                                            Scavenger                                                                     to                                                                            Benzo-                                                                        cyclo-              Conversion                                                                            Selectivity                               Scavenger                                                                             butene    Diluent   (Percent)                                                                             (Percent)                                 ______________________________________                                        T-Butyl 0.14      Methylene 75.6    78                                        Alcohol           chloride                                                    Epichloro-                                                                            1.0       Methylene 85.1    80                                        hydrin            Chloride                                                    Triethyl-                                                                             0.10      Methylene 78.8    85                                        amine             Chloride                                                    Sodium  1.0       Methylene 44.8    80                                        Methylate         Chloride                                                    ______________________________________                                    

Table V indicates that a high selectivity of the reaction tomonobrominated benzocyclobutenes is obtained by the method of thepresent invention using various scavengers instead of complexing agentsor water.

What is claimed is:
 1. A method of preparing monohalogenatedcyclobutarenes consisting essentially of halogenating a cyclobutarenewith a halogenating agent in the presence of an organic complexingagent, an acid scavenger, or water.
 2. The method of claim 1 wherein thehalogenating agent is a brominating agent or a chlorinating agent. 3.The method of claim 2 wherein the halogenating agent is a brominatingagent.
 4. The method of claim 3 wherein the cyclobutarene isbenzocyclobutene.
 5. The method of claim 4 wherein the chlorinatingagent is selected from the group consisting of molecular chlorine,N-chlorosuccinimide and t-butyl hypochlorite.
 6. The method of claim 5wherein the chlorinating agent is molecular chlorine.
 7. The method ofclaim 1 wherein the cyclobutarene is diluted in an appropriatenonreacting diluent.
 8. The method of claim 7 wherein the diluent isselected from the group consisting of methylene chloride, ethylenedichloride, chloroform, carbon tetrachloride, bromochloromethane,hexane, acetic acid, methanol, and water.
 9. The method of claim 8wherein the diluent is methylene chloride.
 10. The method of claim 1wherein the organic complexing agent has an electron donor capacityequal to or slightly greater than the electron donor capacity of thecyclobutarene.
 11. The method of claim 10 wherein the organic complexingagent is selected from the group consisting of saturated aliphaticalcohols and diols having less than 10 carbon atoms; aliphatic polymericdiols having an average molecular weight ranging from about 100 to about15,000; saturated aliphatic ethers having less than 10 carbon atoms;saturated cyclic ethers: saturated quaternary ammonium salts: saturatedcarboxylic acids and their anhydrides having less than 10 carbon atoms;dimethyl formamide; dimethyl sulfoxide: and mixtures of these complexingagents.
 12. The method of claim 11 wherein the organic complexing agentis selected from the group consisting of saturated aliphatic alcoholsand diols having less than 10 carbon atoms: aliphatic polymeric diolshaving an average molecular weight ranging from about 100 to about15,000: saturated aliphatic ethers having less than 10 carbon atoms;saturated cyclic ethers; and saturated quaternary ammonium salts. 13.The method of claim 11 wherein the organic complexing agent is selectedfrom the group consisting of saturated aliphatic alcohols and diolshaving less than 10 carbon atoms: aliphatic polymeric diols having anaverage molecular weight ranging from about 100 to about 15,000:saturated aliphatic ethers having less than 10 carbon atoms: andsaturated cyclic ethers.
 14. The method of claim 11 wherein the organiccomplexing agent is selected from the group consisting of saturatedaliphatic alcohols and diols having less than 10 carbon atoms; aliphaticpolymeric diols having an average molecular weight ranging from about100 to about 15,000; and saturated aliphatic ethers having less than 10carbon atoms.
 15. The method of claim 11 wherein the organic complexingagent is selected from the group consisting of saturated aliphaticalcohols and diols having less than 10 carbon atoms, and saturatedaliphatic ethers having less than 10 carbon atoms.
 16. The method ofclaim 15 wherein the organic complexing agent is selected from the groupconsisting of methanol and ethylene glycol ethyl ether.
 17. The methodof claim 16 wherein the organic complexing agent is methanol.
 18. Themethod of claim 6 wherein the mole ratio of the cyclobutarene to theorganic complexing agent or water ranges from about 0.05:1 to about4.0:1.
 19. The method of claim 1 wherein the acid scavenger reactsreadily with hydrogen halide but does not react readily with thehalogenating agent.
 20. The method of claim 6 wherein the acid scavengeris organic.
 21. The method of claim 20 wherein the organic acidscavenger is selected from the group consisting of epoxides having lessthan 10 carbon atoms; aliphatic tertiary alcohols having less than 10carbon atoms: alkali metal and alkali earth metal salts of aliphaticalcohols having less than 10 carbon atoms: aliphatic primary, secondary,and tertiary amines: heterocyclic compounds: and triarylphosphines. 22.The method of claim 21 wherein the organic acid scavenger is selectedfrom the group consisting of epoxides having less than 10 carbon atomsand tertiary amines.
 23. The method of claim 22 wherein the organic acidscavenger is selected from the group consisting of epichlorohydrin andtriethylamine.
 24. The method of claim 1 wherein the acid scavenger isinorganic.
 25. The method of claim 24 wherein the inorganic acidscavenger is selected from the group consisting of alkali metal andalkali earth metal salts of alcohols and carboxylic acids, alkali metaland alkali earth metal bases, and carbonates and bicarbonates of alkalimetal and alkali earth metals.
 26. The method of claim 25 wherein theinorganic acid scavenger is sodium methylate.
 27. The method of claim 1wherein the mole ratio of the cyclobutarene to the acid scavenger rangesfrom about 0.5:1 to about 2.0:1.
 28. The method of claim 1 wherein theweight ratio of diluent to the cyclobutarene ranges from about 0.5:1 toabout 20:1.
 29. The method of claim 1 wherein the mole ratio of thehalogenating agent to the cyclobutarene ranges from about 0.90:1 toabout 1.10:1.
 30. The method of claim 1 wherein the selectivity of thereaction to monohalogenated cyclobutarenes ranges from about 75 molepercent to about 95 mole percent.
 31. The method of claim 1 wherein thehalogenation reaction is maintained at atmospheric pressure.
 32. Themethod of claim 31 wherein the halogenating agent is added continuouslyor periodically during the bromination step.
 33. The method of claim 32wherein the temperature is maintained in the range of about 25° C. toabout 60° C.
 34. The method of claim 1 wherein the mole ratio of thecyclobutarene to the complexing agent or water ranges from about 0.05:1to about 6.0:1.